In many imaging systems, such as imagesetters or platesetters, a movable optical carriage is used to displace a laser system or other imaging source in a slow scan direction along a curved or planar media support surface (e.g., flatbed, internal drum, external drum, or other support surface). The imaging source exposes a supply of recording media supported on, and held against, the media support surface. Generally, the imaging source includes an optical system for scanning one or more laser or other radiation beams modulated by an information signal over the recording media to record an image onto the recording media. For example, in an internal drum recording system, the imaging source may include a beam deflection assembly, comprising a deflector element (e.g., a mirror) and a spin motor for rotating the deflector element, wherein the beam deflection assembly deflects an imaging beam generated by a radiation source across the recording media.
The recording media to be imaged by an imaging system is commonly supplied in web form or in discrete sheets or plates. The recording media may comprise a photosensitive, radiation sensitive, thermally sensitive, or other type of imageable material. In a given imaging system, different size recording media may be used for different applications, depending on such factors as the type of image to be scanned, etc. Regardless of the size of the recording media that is used, however, it is important to hold the recording media firmly against the media support surface. Any lifting of the recording media away from the media support surface may result in an out of focus image on the recording media. This may be due, for example, to the fixed and short focal depth of the imaging beam, as well as other factors.
In many imaging systems, a vacuum system is employed to hold the recording media against the media support surface. Commonly, the vacuum system operates by drawing a vacuum through a plurality of vacuum ports, disposed over the media support surface, which draw air from a plurality of constant cross-section (e.g., constant depth and width) vacuum grooves. When a single vacuum source is used to draw air from each of the vacuum ports, larger recording media is held more firmly against the media support surface than smaller recording media because the larger recording media covers more of the vacuum ports or grooves. Analogously, smaller recording media is held less firmly against the media support surface than larger recording media because of the air loss through the vacuum ports or grooves not covered by the smaller recording media, or a drop in vacuum caused by constant cross-section groove drag and loss. This may become problematic when the recording media comprises a flat aluminum plate or the like which is to be held on a curved media support surface, since the vacuum forces required to hold the aluminum plate firmly against the media support surface may be quite substantial.
Many attempts have been made to provide a vacuum system capable of providing a sufficient vacuum for different size recording media. One such technique is described in commonly assigned US patent applications SN: 08/867,128, filed Jun. 4, 1997, entitled "Method and Apparatus For Selectively Drawing Air From A Plurality Of Vacuum Channels, now U.S Pat. No. 6,047,733, and Ser. No. 08/868,526, filed Jun. 4, 1997, entitled "Method and Apparatus For Holding Recording Media Onto A Media Support Surface, now U.S. Pat. No. 6,133,396, both incorporated herein by reference. This technique employs a sequencing manifold to selectively draw a vacuum through a plurality of individually addressable vacuum sections depending on the size of the recording media. Unfortunately, such a vacuum and manifold system, although quite effective, is generally highly complex and expensive to produce.